1. Field of the Invention
The invention of the present application relates to an articulated construction vehicle such as a wheel loader or self-propelled compacting machine, which is provided with a self-propelled body with a front frame and a rear frame connected together rotatably relative to one another in a horizontal direction such that the self-propelled body can be articulated and is also provided with body-articulating hydraulic cylinders as hydraulic actuators for articulating the self-propelled body.
2. Description of the Related Art
Self-propelled construction vehicles include special-purpose vehicles that as unseen in general construction vehicles, their self-propelled bodies are articulated to make them go in a curve upon making them turn. Such construction vehicles will be called “articulated construction vehicles” herein, and as representative examples, articulated wheel loaders can be mentioned. In such an articulated construction vehicle, a self-propelled body is divided into a front frame supported by front wheels and a rear frame supported by rear wheels, and both the frames are connected rotatably relative to one another in a horizontal direction such that the self-propelled body can be articulated. Also provided as hydraulic actuators for articulating the self-propelled body are body-articulating hydraulic cylinders selectively extendable or retractable such that the front frame can be driven to rotate relative to the rear frame in the horizontal direction.
In addition to the body-articulating hydraulic cylinders, the articulated construction vehicle is also provided with a pilot-operated directional control valve and a control device to construct a steering device. The pilot-operated directional control valve controls a direction and amount of pressure oil to be fed to each of these pilot-operated directional control valves, and by operator's operation, the control device outputs a pilot signal to the corresponding one of signal-receiving ports of the pilot-operated directional control valve to switch an operated position and opening area of the directional control valve. Roughly dividing such hydraulic steering devices depending on their operation means through which operators operate the steering devices, there are the wheel-type that each steering device is provided with a steering wheel to be rotationally operated and the lever type that each steering device is provided with a lever to be tilt(pivot)-operated.
It obviously becomes necessary to make the articulated construction vehicle travel when performing work, for example, when performing digging work or loading work of excavated earth and/or sand on a truck by a wheel loader at a construction site. Because such a wheel loader can travel at 35 km or so per hour, it would be convenient if the wheel loader can be designed to permit traveling to go and come on general roadways in addition to traveling to perform such work. In fact, an articulated construction vehicle is allowed under regulations to travel on general roadways. When traveling on general roadways, however, it is required under regulations to operate a steering system through a steering wheel. For this reason, wheel-type steering systems have heretofore been generally adopted for articulated construction vehicles.
A wheel-type, vehicle steering system provided with hydraulic cylinders as hydraulic actuators for steering as mentioned above is disclosed, for example, in Patent Document JP-B-2919051. Considering the features of such a wheel-type, vehicle steering system, operation of a steering wheel is rotational operation as opposed to tilt operation of a lever, and the steering wheel is extremely large in the amount of maximum operation compared with the lever. This steering system can, therefore, perform fine operation. Concerning the operation of a steering system, traveling on a general roadway requires to frequently perform fine operation such as making the vehicle turn subtly although it is unnecessary to frequently perform such operation as making the vehicle go in a large curve. An articulated construction vehicle provided with a wheel-type steering system is hence suited for traveling on general roadways. The articulated construction vehicle provided with such a wheel-type steering system can be driven or operated with the same maneuvering feeling as general cars, and in this respect, is also suited for traveling on general roadways. See Patent Document JP-B-2919051 (pages 1-3, FIGS. 1-2).
An articulated construction vehicle with a wheel-type steering system adopted in Patent Document JP-B-2919051 is suited for traveling on general roadways as described above, but is not fully suited for work at construction sites. An explanation will be made about its reasons. Firstly, it is necessary to frequently perform large operation to make the vehicle body go in a curve when performing work at a construction site. As mentioned above, operation of a steering wheel is extremely large in the amount of maximum operation compared with operation of a lever. When digging work is performed by a wheel loader at a narrow working site, for example, it often becomes necessary to make the vehicle body go in a curve leftwards and rightwards to maximum extent. In such a case, the steering wheel has to be rotated counterclockwise four full turns or clockwise two full turns. With the articulated construction vehicle with the steering system of this type, large operation which requires a large operation amount as described above is frequently performed so that an operation-related considerable load is applied to an operator to cause operator fatigue.
Secondly, a steering wheel is suited for its operation while being held with both hands. However, when traveling while performing work as in the case of performing digging work by a wheel loader, one or more hydraulic actuators relating to the work have to be also operated in addition to the body-articulating hydraulic cylinders. This unavoidably requires to operate the steering wheel with only one hand, so that the operation is awkward. Further, such working hydraulic actuators are generally operated by control means such as levers, and therefore, are manually operated by the operation means which perform movements of different kind from that of the steering wheel. When traveling while performing work, it is therefore necessary to perform operation of different kind from that of the steering wheel, such as tilt-operation of a control means like a lever with one hand while rotationally operating the steering wheel with the other hand. This also makes the operation awkward. For these reasons, the articulated construction vehicle with the wheel-type steering system adopted therein is not considered to be suited for work at construction sites.